A hydraulic motor has, in general, a shaft rotatably supported in a casing. The shaft in turn is coupled to a cylinder block which is rotatable therewith. The cylinder block is formed with a plurality of cylinder bores which are circumferentially spaced apart from one another and in parallel to the shaft. Each of the cylinder bores has a piston slidably inserted therein, thus defining a cylindrical chamber between the piston and an end of the cylinder bore. Also, the above mentioned shaft has a thrust ball coupled thereto, which in turn has a shoe inserted therein so as to be inclined with respect to the shaft by extending along a swash plate. The thrust ball and the shoe are designed to be rotatable with the shaft while being energized towards the above mentioned swash plate by a spring. Further, each of the above mentioned pistons has a front end connected with the shoe via a universal coupling thereto. If the pistons are reciprocated by supplying and draining a pressure fluid from a hydraulic pressure source into and out of a pressure receiving chamber of each cylinder bore the above mentioned swash plate and shoe will guide the pistons circumferentially causing the shaft and the cylinder block to rotate together.
Also, a braking apparatus for the above mentioned hydraulic motor has a plurality of movable side frictional plates and a plurality of fixed side frictional plates mounted to the cylinder block and housing, respectively, so that they are alternately mounted with respect to one another in a direction which is parallel to the above mentioned shaft. Opposite the frictional plates is provided a piston that is adapted to be pushed towards a first side of the frictional plates by a spring and to be pushed towards a second side opposite the first side by a pressure fluid in a piston driving pressure receiving chamber. Using the spring to push the piston causes pressure contact between the fixed side frictional plates and the moveable side frictional plates. As a result, the rotational speed of the cylinder is reduced and is eventually rendered incapable of being rotated. In contrast, supplying the piston driving pressure receiving chamber with a pressure fluid causes the piston to move against the spring. As a result, the fixed side frictional plates and the movable side frictional plates are moved apart from one another. Consequently, braking force is not applied to the cylinder block and thus it is capable of being rotated.
In such a braking apparatus for a hydraulic motor in which the non-braking state is established by supplying the piston driving pressure receiving chamber with the pressure fluid and in which braking force is established by draining the pressure fluid from the piston driving pressure receiving chamber, it can be seen that the time required to switch from the non-braking state to the braking state is determined by the distance and the velocity of movement of the piston (i.e., the time required to drain the pressure fluid from the piston driving pressure receiving chamber).
The time required to drain the pressure fluid from the above mentioned piston driving pressure receiving chamber is determined by a difference between a volume of the fluid in the chamber during the non-braking state and a volume of the fluid in the chamber during the braking state, that is, an effective volume in the piston driving pressure receiving chamber and a velocity of draining the pressure fluid out.
On the other hand, in the case of a hydraulic motor for turning an upper vehicle body in a hydraulic power shovel, it has been observed that after the hydraulic motor is stopped by returning an operating valve to a neutral position the hydraulic motor will continue to rotate a number of times due to an inertia of the upper vehicle body.
For this reason, if the braking apparatus is brought into a braking state at the same time the hydraulic motor is stopped, a large braking force will be applied to the hydraulic motor which may damage the hydraulic motor and shock the apparatus. In order to avoid such inconveniences, the braking apparatus must be operated or brought into the braking state a short period of time or delay after the hydraulic motor is intended to be stopped.
Accordingly, it has hitherto been customary to provide the piston driving pressure receiving chamber in a pressure fluid supply circuit thereof with a check valve and a slow return valve that is capable of allowing a pressure fluid to flow smoothly into the piston driving pressure receiving chamber to establish the non-braking state in a short period of time and on the other hand also capable of allowing the pressure fluid in the piston driving pressure receiving chamber to slowly flow out through a restriction (i.e., a small opening) to bring the braking apparatus slowly into the braking state.
By the way, it should be noted that the braking apparatus in the prior art has been so constructed that if a braking force is not generated, the spacing between the fixed side frictional plates and the movable side frictional plates may be regulated by the piston. This will reduce both the difference in position of the piston between the braking state and the non-braking state (a movement distance of the piston) and the effective volume of the piston, thus requiring the above mentioned restriction to have an extremely small opening area so that the piston will move slowly. As a result, the time period up to the point at which the braking state is established will be lengthened.
In other words, if the above mentioned piston movement distance is increased, it follows that the spacing between the fixed side frictional plates and the movable side frictional plates will be increased and rotation or vibration thereof may cause a backlash in either of the fixed side or the movable side frictional plates. This may cause the moveable side frictional plates to be inclined to fit onto the cylinder block or warped, and may damage such frictional plates during the braking state and result in the generation of an insufficient braking force. For these reasons, it has hitherto been customary to leave a small space between the fixed side frictional plates and the movable side frictional plates.
It has been recognized, however, that with these measures, a machining problem occurs when fabricating a structure having the small restriction or aperture as mentioned above. In addition, fluctuation in the area of the aperture arises from limitations of the attainable machining accuracy and the structure is influenced greatly by changes in the fluid temperature of the high pressure fluid. As a result, an increased fluctuation in the period of time required to bring the braking apparatus into the braking state occurs.
It should be noted at this point that if the piston is increased in diameter its effective volume will also be increased. However, this will then require the size of the housing to be increased, thus eventually enlarging the entire hydraulic motor.
Also, there has hitherto been a proposal to resolve the inconveniences in a case where a slow return valve as mentioned above is utilized, whereby there is provided an electromagnetic valve that is designed to establish and to block communication of the piston driving pressure receiving chamber of the braking apparatus with a hydraulic pressure source and a reservoir. Also, a controller is utilized which is designed to electrically energize the electromagnetic valve after a predetermined period of time after the controller has been supplied with a signal indicative of the fact that steps have been taken to stop the hydraulic motor. The electromagnetic valve is electrically energized by the controller to allow the piston driving pressure receiving chamber to communicate with the reservoir to bring the braking apparatus into a desired braking state.
However, such a construction which requires an electromagnetic valve and a controller as mentioned above is very expensive to manufacture.
It is, accordingly, an object of the present invention to provide a braking apparatus for a hydraulic motor, which is capable of slowly shifting from a non-braking state to a braking state, and which does not require a large hydraulic motor and which is relatively less expensive to produce.